A hybrid engine-electric motor type four-wheel drive system is known in conventional vehicle drive system field. In the drive system, either a frond wheel drive-axle or a rear wheel drive-axle (here, the front wheel drive-axle is cited as an example of that, and it will be also called as “front wheels” below) is driven by an internal combustion engine, and the other (here, the rear wheel drive-axle is cited as an example of that, and it will be also called as “rear wheels” below) is driven by a motor as necessary.
Four-wheel drive mode in which both of the engine and motor are used as a power source is performed under heavy-load conditions such as starting, climbing, or the like of the vehicle. Under normal drive mode in which a load condition is relatively light (low load drive zone), two-wheel drive mode is performed only by the engine.
In the conventional hybrid-type four-wheel drive system, a DC motor or AC motor is used as a wheel-drive motor. Conventionally, a power supplies for accessories, for example, an alternator and a battery of 12V or 14V used for general vehicle-mounted electric equipments, are not used for the wheel-drive system. Instead of them, a generator only for the wheel-drive motor (for example, an alternator having a power generation output of 50V or higher) and a high-voltage battery dedicated for the motor are used as power supplies of the motor (for example, Japanese Patent Laid-Open No. 2001-253256).
This is because power supply necessary for driving the wheel drive motor cannot be secured only by the battery of 12V or 14V for accessories (for example, general electric equipments such as starter, light, and air conditioner) and an alternator corresponding to the battery.
In the case of executing electric drive of wheels at the time of starting of a vehicle, at the time of starting the engine when the engine speed is low and the generation power of the alternator for the wheel drive motor is low, the alternator enters a separate excitation mode. The field current is supplied from the high-voltage battery for the motor to an alternator field winding to increase the generation output of the alternator, and the motor is driven by the generation output. When the vehicle speed reaches predetermined speed (for example, 20 km/hour) after starting, the alternator for the wheel drive motor outputs power sufficient to drive the motor, so that the alternator uses the output power as the power supply of the field winding of itself (self excitation mode).
Japanese Patent Laid-Open No. 2000-224709 discloses a technique of mounting a generator according to a high-voltage specification and a main battery of a motor for driving wheels and an auxiliary battery for accessories, driving the motor by the power from the generator according to the high-voltage specification or the main battery, in addition, stepping down the high-voltage power (for example, 250V to 350V) by the generator to a low voltage (12V or 24V) by a DC-DC converter, and supplying the low voltage to the auxiliary battery for accessories.
Further, in this kind of hybrid-type electric four-wheel drive system, as disclosed in Japanese Patent Laid-Open No. 2001-63392, a motor with a regenerating function (motor/generator; MG) is used as driving means for one wheels (for example, rear wheels). During times of braking of the vehicle, the motor functions as the generator to perform regenerative control of charging regenerative power output from the MG into a capacitor dedicated to drive the motor. The power accumulated in the capacitor is supplied to the MG so as to perform auxiliary wheel drive control by using the MG at the time of starting and accelerating on a road having high coefficient of friction (high μ road) such as a normal road or a dry road.